This June 2012 NSA presentation describes methods to exfiltrate data, even from networks that are apparently offline: see the Der Spiegel article The Digital Arms Race: NSA Preps America for Future Battle, 17 January 2015.

﻿Moving Data Through
Disconnected Networks
Delay-Tolerant Networking and
the IC (U//FOUO)
June ;
The overall classification of this briefing is:
TOP SECRET//COMINT//REL TO USA, FVEY
...